1. Field of the Invention
The present invention relates generally to the field of optical filters and sensors. More particularly, it concerns the use of the guided-mode resonance effect occurring through the use of waveguide gratings attached to the endfaces of waveguides such as optical fibers in fields such as optical sensing and communications.
2. Description of Related Art
Resonance anomalies occurring in waveguide gratings (WGGs) have been the subject of current interest for spectral filtering applications [Magnusson and Wang, 1992; Wang and Magnusson, 1993; Wang and Magnusson, 1994; Shin et al., 1998; Tibuleac and Magnusson, 1997; Tibuleac, et al., 2000; Wawro, et al., 2000; Avrutsky, et al., 1989; Boye and Kostuk, 1999; and Rosenblatt, et al., 1997]. Guided-mode resonances (GMRs) occurring in subwavelength WGGs admitting only zero-order propagating diffraction orders yield spectral filters with unique properties such as peak reflectances approaching 100%, narrow linewidths, and low sidebands. Filter characteristics, such as center wavelength, linewidth and sideband behavior, are defined by the waveguide-grating parameters, such as grating period, grating profile, refractive indices, layer thicknesses, and grating fill factor.
Changes in any parameters of the diffractive structure can result in a responsive shift of the reflected or transmitted wavelength band. In general, for spectral filtering applications, the most stable GMR structure is sought to prevent an unwanted resonance shift due to small parameter fluctuations. However, for spectroscopic sensing applications, it is desirable to enhance the resonance instability to create a device that will respond to very small parameter changes. This type of device can be utilized, for example, to detect very small changes in the refractive index or thickness of a media being evaluated in biomedical, industrial or environmental sensing applications. Implementation of the guided-mode resonance effect for optical sensing using planar waveguide grating structures and free-space propagating incident waves has been proposed in previous publications [Wang and Magnusson, 1993; Shin et al., 1998].
Experimental fabrication of waveguide gratings utilizing the GMR effect has primarily been restricted to planar WGGs with an incident beam that is propagating in free space. Experimental results for 1-D grating GMR filters incorporate single layer and multilayer reflection filter designs, including a TM polarization reflection filter utilizing the Brewster effect [Magnusson, et al., 1998]. Double layer GMR filter efficiencies as high as 98.5% have been reported by Liu, et al. for TE incident polarization [Liu, et al., 1998]. GMR crossed grating structures (2-D grating filters) have been experimentally fabricated by Peng and Morris [Peng and Morris, 1996], with a reported filter efficiency of 60%. Norton et al. [Norton, et al., 1998] investigated the dependence of lineshape and tunability in central wavelength and resonant angle position on grating parameters.
Chen [Chen, 1988] reports a theoretical design incorporating a diffraction grating on an optical fiber endface that is used to excite higher order modes in multimode optical fibers. Wang et al. [Wang, et al., 1995] reports a fiber optic proximity sensor design incorporating a diffraction grating on a fiber endface. However, the diffraction gratings reported in these two references do not have waveguide properties, and, consequently, do not exhibit the GMR effect.
A biosensor is an analytical device that integrates an immobilized biologically sensitive material (analyte), such as enzyme, antibody, DNA, cells, or organic molecules, with an electrochemical, piezoelectric, optical or acoustic transducer to convert a biochemical response into a signal for measurement, interpretation, or control. Electrochemical and optical sensors are most widely used. Optical biosensors can provide fast, accurate, and safe analyte detection. Current fiber-optic sensor technology applies fluorescence, total internal reflection, intensity reflection, and surface-plasmon resonances.
The surface plasmon resonance (SPR) effect, is a widely used optical detection method that is highly sensitive to changes in the optical properties (refractive index, monolayer thickness) at the sensor surface. The term surface plasmon (SP) refers to an electromagnetic field charge-density oscillation that can occur at the surface of a conductor. An SP mode can be resonantly excited by parallel-polarized (TM) incident light. Conventional surface plasmon sensors include a prism or diffraction grating for phase matching of the incident and SP waves; commercial systems employ bulk optical components. Fiber-optic SPR sensors have been reported; in these a metal sleeve is deposited on the side of the fiber to which the analyte is contacted. A drawback of the SPR technology is the inherently large linewidth; typically Δλ˜50 nm. Therefore, a sensor utilizing the GMR effect that would provide smaller linewidths would exhibit a significant resolution dynamic-range advantage over SPR sensors.